Forced swim stressor: Trends in usage and mechanistic consideration

The acquired immobility response during the "forced swim test (FST)" is not a rodent model of depression, but the test has some validity in predicting a compound's antidepressant potential. Nevertheless, 60% of the about 600 papers that were published annually the past 2 years label the rodent's immobility response as depression-like behaviour, but the relative contribution per country is changing. When the Editors-in-Chief of 5 journals publishing most FST papers were asked for their point of view on labelling immobility as depression-like behaviour and despair, they responded that they primarily rely on the reviewers regarding scientific merit of the submission. One Editor informs authors of the recent NIMH notice (https://grants.nih.gov/grants/guide/notice-files/NOT-MH-19-053.html) which encourages investigators to use animal models "for" addressing neurobiological questions rather than as model "of" specific mental disorders. The neurobiological questions raised by use of the FST fall in two categories. First, research on the role of endocrine and metabolic factors, with roots in the 1980s, and with focus on the bottom-up action of glucocorticoids on circuits processing salient information, executive control and memory consolidation. Second, recent findings using novel technological and computational advances that have allowed great progress in charting top-down control in the switch from active to passive coping with the inescapable stressor executed by neuronal ensembles of the medial prefrontal cortex via the peri-aquaductal grey. It is expected that combining neural top-down and endocrine bottom-up approaches will provide new insights in the role of stress-coping and adaptation in pathogenesis of mental disorders.